Transcutaneous energy transfer device with magnetic field protected components in secondary coil

ABSTRACT

The invention provides a transcutaneous energy transfer device having an external primary coil and an implanted secondary coil inductively coupled to the primary coil, electronic components subcutaneously mounted within the secondary coil and a mechanism which reduces inductive heating of such components by the magnetic field of the secondary coil. For one embodiment of the invention, the mechanism for reducing inductive heating includes a cage formed of a high magnetic permeability material in which the electronic components are mounted, which cage guides the flux around the components to prevent heating thereof. For an alternative embodiment of the invention, a secondary coil has an outer winding and either a counter-wound inner winding or an inner winding in the magnetic field of the outer winding. For either arrangement of the inner coil, the inner coil generates a magnetic field substantially canceling the magnetic field of the outer coil in the area in which the electronic components are mounted.

RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/110,607 filed Jul. 6,1998 entitled “TET WITHMAGNETIC FIELD PROTECTED COMPONENTS IN SECONDARY COIL,” and naming asinventors Fred Zarinetchi and Robert M. Hart, now currently pending.

[0002] The following commonly-owned application is related to thepresent application and its disclosure is incorporated by reference inthe present application:

[0003] U.S. patent application entitled “MAGNETIC SHIELD FOR PRIMARYCOIL OF TRANSCUTANEOUS ENERGY TRANSFER DEVICE”, Ser. No. 09/110,608,filed Jul. 6, 1998, naming as inventors Fred Zarinetchi and Steven J.Keville, and now pending.

FIELD OF THE INVENTION

[0004] This invention relates to transcutaneous energy transfer (TET)devices and, more particularly, to such a device which includes amechanism for protecting components mounted within a secondary coil frommagnetic field induced heating.

BACKGROUND OF THE INVENTION

[0005] Many medical devices are now designed to be implantable includingpacemakers, defibrillators, circulatory assist devices, cardiacreplacement devices such as artificial hearts, cochlear implants,neuromuscular simulators, biosensors, and the like. Since almost all ofthe active devices (i.e., those that perform work) and many of thepassive devices (i.e., those that do not perform work) require a sourceof power, inductively coupled transcutaneous energy transfer (TET) andinformation transmission systems for such devices are coming intoincreasing use. These systems consist of an external primary coil and animplanted secondary coil separated by an intervening layer of tissue.

[0006] One problem encountered in such TET systems is that the bestplace to locate control circuitry for converting, amplifying andotherwise processing the signal received at the secondary coil beforesending the signal on to the utilization equipment is within thesecondary coil itself. However, there is also a significant magneticfield in the secondary coil resulting from the current induced therein,which field can induce heating of the components, particularly metalliccomponents. As a minimum, such heating can influence the performance ofvarious components, and in particular interfere with the desired uniformpower applied to the equipment. In a worst case, the heating can besevere enough to cause damage or destruction to the components which canonly be repaired or replaced through an invasive surgical procedure.Such heating can also cause injury or discomfort to the patient in whichthe components have been implanted.

[0007] Heretofore, in order to avoid such heating, it has either beennecessary to be sure that the signal induced in the secondary coil isnot sufficient to generate a magnetic field which would causepotentially damaging heating of the components or to mount thecomponents at a less convenient location. The former is undesirablebecause it is generally not possible to eliminate significant heating ofthe components while still operating the device at required energylevels, and the later solution is not desirable since the output signalfrom the secondary coil can reach 500 volts and above at an operatingfrequency that can be in excess of 100 kHz. It is preferable that suchhigh voltage signal not pass extensively through the body and it isdifficult to provide good hermetically sealed connectors for signals atthese voltages. It is therefore preferable that an auxiliary signalprocessing module, which may reduce the voltage to a value in theapproximately 20 volt range, be included as close to the secondary coilas possible, a position inside the secondary coil being ideal for thispurpose.

[0008] A need therefore exists for an improved technique for use withTET devices so as to enable at least selected electronic components tobe mounted within the secondary coil with minimal heating of suchdevices.

Summary of the Invention

[0009] In accordance with the above, this invention provides atranscutaneous energy transfer device which includes an external primarycoil to which energy to be transferred is applied, an implantedsecondary coil inductively coupled to the primary coil, each of thecoils generating a magnetic field, and electronic componentssubcutaneously mounted within the secondary coil, with a mechanism beingprovided which reduces inductive heating of such components by themagnetic field of the secondary coil. For one embodiment of theinvention, the mechanism for reducing inductive heating includes a cageformed of a high magnetic permeability material in which the electroniccomponents are mounted. The material of such cage is preferably aferromagnetic material such as a ferrite and is preferably sufficientlythick so that magnetic field values in the material are well belowsaturation and so that significant heat dissipation in the material doesnot occur. The material of the cage should however be as thin aspossible while satisfying the above criteria. The cage may be thicker inareas of the cage experiencing high flux density and thinner in otherareas. The cage may also be formed of a layer of ferromagnetic materiallaminated with at least one layer of a low magnetic conductivitymaterial to enhance flux guidance.

[0010] Alternatively, the mechanism may include winding the secondarycoil with a first number N₁ of outer windings and a second number N₂ ofcounter-wound inner windings, N₁ being larger than N₂. N₁, N₂ and thediameters of both the outer and inner windings are selected such thatthe magnetic field caused by the coils in the region of the componentsis reduced sufficiently to prevent significant component heating. For apreferred embodiment, N₁, N₂, and the diameters of the windings areselected so that the magnetic fields caused by the windingssubstantially cancel in the region of the components. For anillustrative embodiment, N₁ is approximately 19, N₂ is approximately 7,and the diameter of the outer winding is approximately 2.5 inches, andthe diameter of inner winding is approximately 1.5 inches.Alternatively, the inner windings may be in the magnetic field of theouter winding, but not electrically connected thereto.

[0011] In another aspect of the invention, a transcutaneous energytransfer device is disclosed. The TET includes an external primary coil;an implantable secondary coil coupled to the primary coil; a cage formedof a high magnetic permeability material located within the secondarycoil to reduce inductive heating of electronic components mountedtherein caused by a magnetic field of the primary and secondary coils,wherein the cage has walls of varying thickness such that a lowest totalmass is achieved without exceeding the saturation density of the cagematerial. In one embodiment, the thickness of the cage walls is aminimum thickness that results in magnetic flux density through the cagewalls is approximately equal to the saturation density.

[0012] In another aspect of the invention, another transcutaneous energytransfer device is disclosed. This TET includes an external primarycoil; an implantable secondary coil coupled to the primary coil; and acage formed of a high magnetic permeability material located within thesecondary coil to reduce inductive heating of electronic componentsmounted therein caused by a magnetic field of the primary and secondarycoils, wherein the cage has a geometry configured to maximizepermeability in flux pathway between the primary and secondary coils. Inone embodiment, the cage has flanges that extend the high permeabilityshield material within the flux pathway.

[0013] In another embodiment of this aspect of the invention, the cageincludes a cylindrical base; a lid shaped in the form of a disk; and theflanges integral with the base. The flanges extend a high magneticpermeable region from base into the magnetic flux pathway. Preferably,the flange is in-line with a shortest flux pathway between the primaryand secondary coils, such as extending from base immediately adjacent tothe secondary coil to guide the magnetic flux lines back toward theprimary coil.

[0014] In another aspect of the invention, a transcutaneous energytransfer device is disclosed. This TET includes an external primarycoil; an implantable secondary coil coupled to the primary coil; and acage formed of a high magnetic permeability material within thesecondary coil to house electronic components. The cage includes a base;and a self-aligning lid.

[0015] In one embodiment of this aspect of the invention, the base iscylindrical and the lid is shaped in the form of a disk. In antherembodiment, the base includes vertical walls. The lid includes anannular recessed shelf circumferentially formed around a mating surfaceof the lid configured to receive the vertical wall of the base.

[0016] In a stiff further aspect of the invention, a transcutaneousenergy transfer device is disclosed. The device includes an externalprimary coil; an implantable housing formed of a substantially lowthermal conductivity medium; a secondary coil, mounted within theimplantable housing, coupled to the primary coil; a cage formed of ahigh magnetic permeability material within the secondary coil to houseelectronic components; and a heat distribution layer thermally coupledto the cage and to an internal surface of the housing. The heatdistribution layer may be comprised of multiple alternating layers ofhigh and low heat conductivity materials.

[0017] In a further aspect of the invention, a transcutaneous energytransfer system is disclosed. The TET includes a primary coil and animplantable secondary coil having an outer first winding having a firstnumber of turns and a first diameter and an inner second winding havinga second number of turns and a second diameter. A method for determiningthe second number of turns, includes the steps of: a) winding the firstwinding with a predetermined number of turns; b) inserting a magneticfield monitoring device in a central region of the secondary coil; c)applying a dc current through the first winding while monitoring amagnetic field in the central region; d) winding the second winding in adirection a direction of the first winding using a wire extension fromthe first winding; e) monitoring, as the second winding is wound in thestep d), a strength of a magnetic field in the central region; and f)stopping the winding of the second winding when the magnetic fieldstrength reaches approximately zero.

[0018] Further features and advantages of the present invention as wellas the structure and operation of various embodiments of the presentinvention are described in detail below with reference to theaccompanying drawings. In the drawings, like reference numerals indicatelike or functionally similar elements. Additionally, the left-most oneor two digits of a reference numeral identifies the drawing in which thereference numeral first appears.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] This invention is pointed out with particularity in the appendedclaims. The above and further advantages of this invention may be betterunderstood by referring to the following description when taken inconjunction with the accompanying drawings, in which:

[0020]FIG. 1 is a schematic semi-block diagram of an exemplary TETsystem.

[0021]FIG. 2 is a cutaway sectional view of the secondary coil andelectronics for a system of the type shown in FIG. 1 in accordance witha first embodiment of the invention.

[0022]FIGS. 3A and 3B are a side-sectional view and a top plane viewrespectively of a secondary coil and electronics in accordance with asecond embodiment of the invention.

[0023]FIG. 3C is a top view for an alternative second embodiment.

[0024]FIGS. 4A, 4B and 4C are side-sectional views of the secondary coiland electronics illustrating the magnetic field lines at the secondaryfor the embodiment shown in FIG. 1, FIG. 2, and FIGS. 3A-3Crespectively.

[0025]FIG. 5 is a cross-sectional view of an alternative embodiment of acage of the present invention having flanges or extensions extendingtherefrom.

[0026]FIG. 6 is a simplified cross-sectional view of a cage inaccordance with one embodiment of the present invention.

[0027]FIG. 7 is a cross-sectional view of an implantable device housinga secondary coil in accordance with an alternative embodiment of thepresent invention.

DETAILED DESCRIPTION

[0028] Referring to FIG. 1, an exemplary TET system 10 is shown whichincludes a primary coil 12 connected to receive an alternating drivesignal from an appropriate drive circuit 14 and a transcutaneouslymounted secondary coil 16 having signal processing electronics 18mounted therein. The signal from electronics 18 is applied to operate autilization device 20 which may for example be a blood pump, artificialheart, defibrillator, or other implanted device requiring power or othersignals applied thereto. As illustrated in FIG. 4A, the magnetic fieldlines generated by the secondary coil 16 pass through the electronics 18for the embodiment shown in FIG. 1, resulting in heating of theelectronics and, in particular, metal portions thereof. This can causeundesired variations in the outputs from this device or, worst case, incomponent failure. In accordance with certain aspects of the presentinvention, FIGS. 2 and 3A-3C illustrate two techniques for reducinginductive heating of such electronic components 18 by the magnetic fieldgenerated by the primary and secondary coils.

[0029] Referring to FIG. 2, the electronics 18 are fully enclosed withina cage 22 formed of a high magnetic permeability material, the materialfor cage 22 preferably having a magnetic permeability in the range ofapproximately 2000-5000. Cage 22 is formed of a base 22 A and a lid 22Bwhich are fitted and held together in a manner known in the art whichminimizes interruption of magnetic field lines passing through the cage.For a preferred embodiment, the material utilized for the cage is MnZnferrite (Phillips 3F3), although other high magnetic permeabilitymaterials now or later developed may be used. The material is preferablyone formulated for high frequency application with low power loss. Sinceit is desirable that cage 22 be as light (have minimal mass) aspossible, the wall thickness (t) of the cage should be no thicker thanis required to protect electronics 18 therein, preferably whileminimizing heat loss. In particular, the thickness should be the minimumthickness for the material which provides magnetic field values in thematerial which are well below saturation, saturation preventing the fluxfrom being guided effectively, and below the level that would lead tosignificant (for example, greater than 100 mw) heat dissipation in thematerial under operating conditions. Saturation in the cage is afunction of magnetic field strength, while heat generation is a functionof both magnetic field strength and frequency. For a ferrite cagematerial, the field strengths and frequencies at which these effectsoccur can be tabulated. With a give set of operating conditions, fieldstrength or flux density increases with decreasing wall thickness. Themagnetic field strength B may be determined by measurement at the topsurface of the cage 22, and the total flux φ^(t) may be determined asthe integral of B over the surface. This flux is guided through the sidewalls of the cage, density being highest in these side walls. Themagnetic flux in the walls may then be calculated as the total fluxdivided by the cross sectional area of the wall: $\begin{matrix}{B_{\max} = \frac{\Phi_{t}}{( {\pi \quad {Dt}} )}} & {{Eq}.\quad (1)}\end{matrix}$

[0030] A thickness t may be chosen so that B_(max) is below thesaturation level. In an alternative embodiment, the thickness t is alsochosen so that the total heat dissipation at the operating frequency isless than 100 mw. In another embodiment, the thickness t is also chosenso that the total heat dissipation at the operating frequency is between50 and 100 mw. In a still further embodiment, the thickness t is alsochosen so that the total heat dissipation at the operating frequency isless than 150 mw. In a another embodiment, the thickness t is alsochosen so that the total heat dissipation at the operating frequency isbetween 70-90 mw.

[0031] For an exemplary embodiment using the material previouslyindicated, wall thickness (t) is approximately 0.06 inches, and thediameter (D) of the cage for this embodiment is approximately 1.9inches. However, these dimensions may vary significantly withapplication, for example the size of secondary coil 16, the electricenergy applied thereto and the like. Further, the thickness of the cageneed not necessarily be sufficient to divert all magnetic flux linesfrom the electronic 18 so long as it is effective to divert sufficientmagnetic flux lines so as to prevent any significant heating of theelectronic components. FIG. 4B illustrates the magnetic field forsecondary coil 16 when a cage 22 of the type shown in FIG. 2 isemployed. From FIG. 4B, it can be seen that the magnetic field isconcentrated in the high permeability material of the cage so that thefield at the electronic components 18 is reduced to nearly zero, withthe magnetic field concentration being higher in the sidewalls of thecage than in the top and bottom walls. Thus, overall cage thickness andweight may in some instances be reduced by making the sidewalls thickerto accommodate the flux therein, with the top and bottom walls of thecage being thinner.

[0032] In one preferred embodiment, the optimal thickness of the cagewall has the lightest weight (lowest mass) without causing excessiveheat dissipation. To have the lightest cage 22, the cage wall thicknessis reduced, resulting in an increase in the magnetic flux density, B. Tosatisfy operation without excessive heat dissipation, B must be keptbelow the saturation density, B_(max), for the selected cage material.Should B_(max) be exceeded, the heat dissipation of the materialincreases dramatically. The combination of these two requirements, then,results in the, desired requirement that B₀=B_(max), where B₀ is theflux density at the highest possible magnetic field strength for thedevice.

[0033] B₀ at any given point along the cage can be manipulated byvarying the cage wall thickness. The design consideration is to reducethe cross sectional area of the cage so that B₀ is less than or equalB_(max) along a substantial portion of the cage wall. An example of analgorithm for this process for a cylindrically symmetric cage includesfirst assuming a uniform thickness for the cage wall. Then, the totalflux for highest power transfer within the cage walls, i.e. φ₀ using therelationship B₀=φ₀/Lt, where L is the perimeter of the cage 22 at thepoint of interest and t is the cage thickness wall. Given that L isdetermined by other design considerations, t can be adjusted so thatB₀=B_(max).

[0034] While for certain embodiments, the material of cage 22 is of aferrite material, other high permeability materials might also beutilized such as, for example, laminated iron materials. For ease offabrication, it may also be desirable to form the cage as three or moredistinct segments, for example a top and bottom disk with a cylinder forthe sidewalls, the disks and sidewalls being held together by a suitableepoxy, solder or other suitable means known in the art. This form offabrication may be particularly desirable where the sidewalls are of adifferent thickness than the top and bottom walls. Preferably, the breakin material continuity is minimized to avoid a significant reduction inthe efficiency at which the magnetic field through the cage. Finally,forming the cage of alternate layers of ferrite material with highthermal and magnetic conductivity and epoxy or other similar materialwith low thermal and magnetic conductivity, results in a cage which ismore anisotropic for magnetic flux flow, and thus provides potentiallybetter flux guidance. In particular, such construction provides a lowerreluctance path for the magnetic flux and magnetic fields through theferrite layers then in a direction perpendicular thereto.

[0035]FIGS. 3A and 3B illustrate an alternative embodiment of theinvention wherein reduced magnetic field at electronic components 18 isachieved by dividing the secondary coil into an outer coil 16A and aninner coil 16B which is counter-wound with the coil 16A so as to providean opposing field. In this embodiment, the two sets of coils areconnected so that the same current flows in each. The relative diametersof the two coils, D₁ and D₂ and the number of turns for the coils, N₁and N₂, are adjusted so that their separate contributions to the totalmagnetic field at the location of components 18 significantly canceleach other. In achieving this objective, the field generated in thecentral region by a flat outer coil 16A is given,by: $\begin{matrix}{B_{2} = \frac{\mu \quad N_{1}}{D_{1}}} & {{Eq}.\quad (2)}\end{matrix}$

[0036] while the field generated by the inner coil 16B in the centralregion of the coil is given by: $\begin{matrix}{B_{2} = \frac{\mu \quad N_{2}}{D_{2}}} & {{Eq}.\quad (3)}\end{matrix}$

[0037] When the strengths of these two fields are selected to be equal,the ratio between the outer and inner windings becomes: $\begin{matrix}{\frac{N_{1}}{N_{2}} = \frac{D_{1}}{D_{2}}} & {{Eq}.\quad (4)}\end{matrix}$

[0038] Using these criteria with an illustrative embodiment where D₁=2.5inches, D₂=1.5 inches and N₁=19, a value for N₂=11.4 would be obtained.However, for this implementation, because the inner coil was slightlyelongated along the field direction, a preferred value for N₂ was foundto be 7. Therefore, while the four values (the N and D values) can becalculated to achieve the desired magnetic field cancellation for agiven application, it has been found to be easier and more accurate toselect the parameters empirically, for example by selecting three of theparameters to achieve substantial field cancellation and then adjustingthe fourth parameter until the field at the components 18 has beenreduced so that there is no significant heating of the metal componentsthereof. Thus, assuming the other three values are given, an N₂ might bedetermined as follows:

[0039] 1. Wind the outer coil with the predetermined number of turns (19in the example given).

[0040] 2. Insert a magnetic field monitoring device such as a gaussmeter in the central region of the coil.

[0041] 3. Apply a dc current through the outer coil and monitor themagnetic field in the central region.

[0042] 4. Locate one of the ends of the outer coil and using the wireextension from this coil, start winding the inner coil in the oppositedirection of the outer coil.

[0043] 5. As the inner coil is wound, monitor the strength of themagnetic field in the central region. Stop winding the inner coil whenthis field reaches zero.

[0044] Other empirical procedures might similarly be used fordetermining the parameter values in order to achieve substantial fieldcancellation at the center of the coil.

[0045]FIG. 4C illustrates a magnetic flux pattern which might beachieved with the winding pattern of FIGS. 3A-3B. From FIG. 4C, it canbe seen that the magnetic field at components 18 can be reduced tosubstantially zero utilizing this technique. However, since thistechnique involves significant signal cancellation in the secondarycoil, it results in a reduced energy transfer efficiency for the device.It may therefore not be suitable for use in applications where a highenergy transfer efficiency is required.

[0046]FIG. 3C illustrates an alternative to the embodiment shown in FIG.3B which to some extent reduces the loss of energy transfer efficiencyresulting from the counter flowing current in coil 16B. For thisembodiment, coil 16B′ is a few turns of wire, for example one or two,having the diameter D₂ but not electrically connected to the windings16A′. The magnetic flux resulting from current flow in windings 16A′induce a current in winding 16B′ which in turn produce a magnetic fieldcountering that generated by that winding 16A. By suitable selection ofboth the number and diameter of the windings 16A′ and 16B′, fieldcancellation such as that shown in FIG. 4C can be achieved.

[0047] In an alternative embodiment, the cage of the present inventionis configured to increase the permeability in a flux pathway between theprimary and secondary coils that is closest to the secondary coil. Inone embodiment this is achieved by extending the high permeabilityshield material of cage 22 to a location within the flux pathway. Thisincreases the total permeability of the flux pathway with acorresponding increase in coupling between the primary and secondarycoils.

[0048] An exemplary implementation of this alternative embodiment of thecage is illustrated in FIG. 5. As shown therein, cage 500 includes abase 502 and a lid 504. In this illustrative embodiment, base 502 iscylindrical while lid 504 is shaped in the form of a disk. Base 502includes and integral flange 506 that extends the cage material frombase 502 into the magnetic flux pathway 508.

[0049] To increase the coupling between the primary and secondary coils,flange 506 is preferably in-line with the shortest flux pathway betweenthe primary and secondary coils. In other words, flange 506 extends frombase 502 immediately adjacent to secondary coil 512 to guide themagnetic flux lines back toward the primary coil. The extent to whichthe flange 506 extends away from base 502 is based on the mass andvolume limitations of the device. However, flange 506 preferably doesnot have a thickness less than that which would cause saturation of themagnetic material, particularly at highest primary field strengths.

[0050] It should be understood that flange 506 may be the same ordifferent high permeability material as base 502. In devices whereinflange 506 and cage 22 are of the same material, flange 506 and base 502are preferably formed as a unitary device. However, in alternativeembodiments, flange 506 may be attached to base 502 using well-knowntechniques.

[0051]FIG. 6 is a simplified cross-sectional view of an alternativeembodiment of a cage 600. Cage 600 includes a base 602 and anself-aligning or interlocking lid 604. In this illustrative embodiment,base 602 is cylindrical while lid 604 is shaped in the form of a disk.Base 602 includes vertical walls 606 onto which lid 604 is attached,typically by bonding.

[0052] Lid 604 includes an annular recessed shelf 608 circumferentiallyformed around mating surface 610 of lid 604. Shelf 608 is configured toreceive vertical wall 606 of base 602 thereby preventing relative motionof lid 604 and base 602 during assembly. It should be understood thatother features may be used to align and/or secure lid 604 with base 602,such as by threading, etc. Alternatively, mating surface 610 of lid 604may have a track or groove configured to accept base vertical wall 606.It should also be noted that annual recessed shelf 608 may be configuredas a square or any other shape to accept base 602.

[0053]FIG. 7 is a cross-sectional view of an implantable device 702housing a secondary coil 704. Cage 706 is positioned within secondarycoil 704, as described above. Cage 706 serves to house electroniccomponents 708 that generate heat, illustrated by arrows 710. Inaccordance with this embodiment of the present invention, heat inimplanted device 702 is distributed to avoid localized high temperatureregions as well as tissue necrosis.

[0054] Housing 702 is constructed of a relatively low thermalconductivity medium such as potting epoxy. Cage 706 preferably includesflange 712 similar to the flange described above with reference to FIG.6. In accordance with this embodiment of the present invention, a heatdistribution layer 714 is thermally coupled to cage 706 and housing 702.Heat generated by electronics 708 is transferred via conduction to cage706. To facilitate the transfer of such heat into distribution layer 714good thermal contact is maintained between distribution layer 714 andcage 706 using a thermally conductive medium. In addition, the largestcontact area between distribution layer 714 and cage 706 suitable forthe implemented design is implemented. For example, in the embodimentillustrated in FIG. 7, flange 712 provides base 715 with the greatestarea over surface 720 as compared to other surfaces of cage 706.Accordingly, distribution layer 714 is thermally coupled to cage 706 atsurface 720. Similarly, distribution layer 714 is preferably thermallycoupled to a substantial area of housing 702. This provides for thesubstantially uniform distribution of heat along housing 702. In theembodiment illustrated in FIG. 7, distribution layer 714 is thermallycoupled to the entire housing 702 other than the portion of housing 702adjacent bottom surface 720 of cage 706.

[0055] In one embodiment, distribution layer 714 has a high thermalconductivity and a low magnetic permeability and low electricalconductivity. For example, in one preferred embodiment, distributionlayer 714 is formed from alumina powder suspended in an epoxy medium.The thickness of the distribution layer 714 is preferably in the rangeof 2-10 mm, although other thicknesses and materials may be used for agiven application. In an alternative embodiment, distribution layer 714is comprised of multiple alternating layers of high and low heatconductivity materials.

[0056] While specific embodiments have been disclosed above for reducingthe magnetic field at the center of a TET device secondary coil so as topermit device electronics to be mounted in the coil without excessiveheating, it is to be understood that other techniques for achieving thisobjective and/or other variations on the embodiments disclosed arewithin the contemplation of the invention. Thus, while the invention hasbeen particularly shown and described above with reference to preferredembodiments, the foregoing and other changes in form and detail may bemade therein by one skilled in the art while still remaining within thespirit and scope of the invention which is to be defined only by thefollowing claims.

[0057] Further information is described in currently pending U.S. patentapplication Ser. No. 09/110,607 filed Jul. 6, 1998 entitled “TET WITHMAGNETIC FIELD PROTECTED COMPONENTS IN SECONDARY COIL,” and naming asinventors Fred Zarinetchi and Robert M. Hart, now currently pending,incorporated herein by reference in its entirety.

What is claimed is:
 1. A transcutaneous energy transfer devicecomprising: an external primary coil to which energy to be transferredis applied; an implanted secondary coil inductively coupled to saidprimary coil, each of said coils producing a magnetic field; andelectronic components subcutaneously mounted within said secondary coil,characterized by the inclusion of a mechanism which reduces inductiveheating of said components by said magnetic field.
 2. The transcutaneousenergy transfer device of claim 1, wherein said mechanism includes acage formed of a high magnetic permeability material in which saidelectronic components are mounted.
 3. The transcutaneous energy transferdevice of claim 2, wherein said material is a ferromagnetic material. 4.The transcutaneous energy transfer device of claim 2, wherein thematerial of said cage is sufficiently thick so that magnetic fieldvalues in the material are well below saturation.
 5. The transcutaneousenergy transfer device of claim 2, wherein the material of said cage issufficiently thick so that significant heat dissipation in the materialdoes not occur.
 6. The transcutaneous energy transfer device of claim 2,wherein the material of said cage is as thin as possible while stillbeing sufficiently thick so that magnetic field values in the materialare well below saturation and significant heat dissipation in thematerial does not occur.
 7. The transcutaneous energy transfer device ofclaim 6, wherein said cage is thicker in areas of the cage experiencinghigh flux density and thinned in other areas.
 8. The transcutaneousenergy transfer device of claim 6, wherein said cage is formed of alayer of ferromagnetic material laminated with at least one layer of lowmagnetic conductivity material.
 9. The transcutaneous energy transferdevice of claim 1, wherein said mechanism includes said secondary coilbeing wound with a first number N₁ of outer windings and a second numberN₂ of counter-wound inner windings, N₁ being larger than N₂, N₁, N₂ andthe diameters of both the outer and inner windings being selected suchthat the magnetic field caused by the coils in the region of saidcomponents is reduced sufficiently to prevent significant componentheating.
 10. The transcutaneous energy transfer device of claim 9,wherein N₁, N₂ and the diameters of the windings are selected so thatthe magnetic fields caused by the windings substantially cancel in theregion of said components.
 11. The transcutaneous energy transfer deviceof claim 9, wherein N₁ is approximately 19, N₂ is approximately seventhe diameter of the outer winding is approximately 2.5″ and the diameterof the inner winding is approximately 1.5″.
 12. The transcutaneousenergy transfer device of claim 1, wherein said mechanism includes saidsecondary coil being wound with a first number N₁ of outer windings andincluding a second number N₂ of inner windings in the magnetic field ofthe outer windings, N₁ being larger than N₂, N₁, N₂ and the diameters ofboth the outer and inner windings being selected such that the magneticfield caused by the coils in the region of said components is reducedsufficiently to prevent significant component heating.
 13. Atranscutaneous energy transfer device comprising: an external primarycoil; an implantable secondary coil coupled to said primary coil; a cageformed of a high magnetic permeability material located within saidsecondary coil to reduce inductive heating of electronic componentsmounted therein caused by a magnetic field of said primary and secondarycoils, wherein said cage has walls of varying thickness such that alowest total mass is achieved without exceeding the saturation densityof said cage material.
 14. The transcutaneous energy transfer device ofclaim 13, wherein said thickness of said cage walls is a minimumthickness that results in magnetic flux density through said cage wallsis approximately equal to said saturation density.
 15. A transcutaneousenergy transfer device comprising: an external primary coil; animplantable secondary coil coupled to said primary coil; a cage formedof a high magnetic permeability material located within said secondarycoil to reduce inductive heating of electronic components mountedtherein caused by a magnetic field of said primary and secondary coils,wherein said cage has a geometry configured to maximize permeability influx pathway between the primary and secondary coils.
 16. Thetranscutaneous energy transfer device of claim 15, wherein said cage hasflanges that extend the high permeability shield material within theflux pathway.
 17. The transcutaneous energy transfer device of claim 16,wherein said cage comprises: a cylindrical base; a lid shaped in theform of a disk; and said flanges integral with said base, said flangesextending a high magnetic permeable region from base into the magneticflux pathway.
 18. The transcutaneous energy transfer device of claim 17,wherein said flange is in-line with a shortest flux pathway between saidprimary and secondary coils.
 19. The transcutaneous energy transferdevice of claim 17, wherein said flange extends from base immediatelyadjacent to said secondary coil to guide said magnetic flux lines backtoward said primary coil.
 20. A transcutaneous energy transfer devicecomprising: an external primary coil; an implantable secondary coilcoupled to said primary coil; a cage formed of a high magneticpermeability material within said secondary coil to house electroniccomponents, wherein said cage is comprised of: a base; and aself-aligning lid.
 21. The transcutaneous energy transfer device ofclaim 20, wherein said base is cylindrical and wherein said lid isshaped in the form of a disk.
 22. The transcutaneous energy transferdevice of claim 20, wherein said base includes vertical walls andwherein said lid includes an annular recessed shelf circumferentiallyformed around a mating surface of said lid configured to receive saidvertical wall of said base.
 23. A transcutaneous energy transfer devicecomprising: an external primary coil; an implantable housing formed of asubstantially low thermal conductivity medium; a secondary coil, mountedwithin said implantable housing, coupled to said primary coil; a cageformed of a high magnetic permeability material within said secondarycoil to house electronic components; and a heat distribution layerthermally coupled to said cage and to an internal surface of saidhousing.
 24. The transcutaneous energy transfer device of claim 23,wherein said heat distribution layer is comprised of multiplealternating layers of high and low heat conductivity materials.
 25. In atranscutaneous energy transfer system including a primary coil and animplantable secondary coil having an outer first winding having a firstnumber of turns and a first diameter and an inner second winding havinga second number of turns and a second diameter, a method for determiningsaid second number of turns, comprising: a) winding said first windingwith a predetermined number of turns; b) inserting a magnetic fieldmonitoring device in a central region of said secondary coil; c)applying a dc current through said first winding while monitoring amagnetic field in the central region; d) winding said second winding ina direction a direction of said first winding using a wire extensionfrom said first winding; e) monitoring, as said second winding is woundin said step d), a strength of a magnetic field in the central region;and f) stopping said winding of said second winding when the magneticfield strength reaches approximately zero.